1. Field of the Invention
This invention concerns Fe-Cr-Ni-Si shape memory alloys with excellent stress corrosion cracking resistance, and in particular, relates to the Fe-Cr-Ni-Si shape memory alloys with excellent stress corrosion cracking resistance, having good shape-memorizing properties, corrosion resistance and intergranular corrosion resistance in high-temperature, high-pressure water for the nuclear power field or in nitric acid for nuclear fuel reprocessing plants.
2. Description of the Prior Art
A ferrous-group shape memory alloy features the property of being restored to its shape prior to plastic deformation when the alloy is subjected to plastic deformation at a specified temperature close to the martensite transformation temperature and then the alloy is heated to a specific temperature over the inverse transformation temperature to its base phase.
By giving plastic deformation to this shape memory alloy at a specified temperature, the crystalline structure is transformed from its base phase into martensite.
As described above, when an alloy which is subjected to plastic deformation is heated to a specified temperature over the inverse transformation temperature to its base phase, martensite is inversely transformed to its original base phase: the alloy shows shape-memorizing properties, by which the said alloy is restored to its original shape prior to undergoing plastic deformation. Many non-ferrous shape-memory alloys are already known as having these shape-memorizing properties. (For example, "Shape Memory Alloys" edited by Hiroyasu Funakubo, Sangyo Tosho 1984)
Among these conventional non-ferrous shape memory alloys, Ni-Ti and Cu shape memory alloys are already being put to practical use. Tube joints, clothing, medical instruments and actuators are manufactured by employing these non-ferrous shape memory alloys. In recent years, technological development has progressed to a point where these shape memory alloys are now applied to a variety of industrial uses.
However, from the viewpoint of application to structural members, remarkable hydride formation occurs when Ni-Ti shape memory alloys are used in high-temperature water. Accordingly, they have been unsuitable for such an environment. Cu-Zn-Al shape memory alloys have insufficient corrosion resistance. Moreover, these non-ferrous shape memory alloys are expensive, and from an economical viewpoint, their use is limited.
Under such circumstances, ferrous-group shape memory alloys which are less expensive than non-ferrous shape memory alloys are being developed. A more extensive scope of application is envisaged for the ferrous-group shape memory alloys as opposed to the non-ferrous shape memory alloys which are restricted in use due to their prohibitive cost. The martensite to which a ferrous-group shape memory alloy is transformed from its base phase by undergoing plastic deformation can be roughly divided into fct (face-centered tetragonal structure), bct (body-centered cubic structure) and hcp (dense hexagonal structure) from the viewpoint of crystalline structure. Ferrous group shape memory alloys that are transformed from their base phase to .epsilon. martensite of dense hexagonal structure by undergoing plastic deformation and that are excellent in corrosion resistance are proposed in JP, A No. 2-77554 (hereinafter referred to as "the first prior art"). That is, the alloys based on the first prior art contain Cr: 5.0-20.0 wt %, Si: 2.0-8.0 wt %, at least one element selected in the group comprising Mn: 0.1-14.8 wt %, Ni: 0.1-20.0 wt %, Co: 0.1-30.0 wt %, Cu: 0.1-0.3 wt %, N: 0.001-0.400 wt %, and have excellent shape-memorizing properties and corrosion resistance.
In JP, A No. 2-301514, alloys containing Cr: 10-17 wt %, Si: 3.0-6.0 wt %, Mn: 6.0-25.0 wt %, Ni: 7.0 wt % or less, Co: 2.0-10.0 wt % and Ti, Zr, V, Nb, Mo, Cu, etc. are proposed as high Mn shape memory alloys with a high Cr content and improved corrosion resistance (hereinafter referred to as "the second prior art").
On the other hand, B.E. WILDE, "CORROSION-NACE (1986), Vol. 42, No. 11, p. 678" can, for example, be cited as regards ferrous-group alloys with excellent stress corrosion cracking resistance. That is, this report shows alloys with excellent stress corrosion cracking resistance in high-temperature water, containing Cr: 17.0-19.0 wt %, Si: 0.35-4.79 wt %, Ni: 8.83-9.07 wt %, Mn: 1.30-1.53 wt %, Cu: 0.009-0.20 wt %, N: 0.011-0.040 wt % and Mo: 0.019-0.21 wt % (hereinafter referred to as "the third prior art").
The shape memory alloys that are used in nitric acid for nuclear fuel reprocessing plants and in high-temperature water (primary cooling water) for light-water reactors must have excellent shape-memorizing properties, intergranular corrosion resistance and stress corrosion cracking resistance. However, of the said prior arts, none can be found that meets this requirement.
The ferrous-group shape memory alloys disclosed in the said first prior art are ferrous-group alloys to which Cr and Si elements are added to improve the shape-memorizing properties and corrosion resistance and also to which at least one element of Mn, Ni, Co and N is added. However, these alloys have the following problems. Though the shape memory alloys show excellent corrosion resistance, this corrosion resistance was evaluated at an atmospheric exposure test over a period of two years and the said intergranular corrosion resistance in nitric acid and stress corrosion cracking resistance in high-temperature water are not always sufficient. As seen in its working examples, the basic alloy types can be roughly divided into the Fe-13Cr-6Si type and the Fe-18Cr-2Si type. The alloys of the former contain an addition of 15.1 wt % or less of Cr and the alloys of the latter contain an addition of 2.8 wt % or less of Si. Accordingly, the improvement in the intergranular corrosion resistance in nitric acid and stress corrosion cracking resistance in high-temperature water envisaged as an effect of the addition of Cr and Si is inadequate.
In the first prior art, the C and N content is limited to 0.1 wt % or less. The study conducted by this inventor and others shows that when alloys with a total C and N content above 0.01 wt % undergo thermomechanical treatment indispensable to raise their shape-memorizing properties (for example, thermomechanical treatment of heating to 500.degree.-700.degree. C. after deformation is given at ambient temperature), the intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high-temperature water are deteriorated by the lack of Cr from the grain boundary due to the precipitation of Cr carbide or Cr nitride at the grain boundary or the segregation of C or N at the grain boundary even if the said precipitated phases do not exist. However, to reduce the total C and N content to 0.1 wt % or less in the alloy composition provided in the same prior art, no means, except the use of expensive raw materials and/or the use of special fusions, can be found with existing manufacturing techniques, resulting in very high cost.
Moreover, in the said first prior art, Co is added as an optional element. However, as described in the working example, the Co content is 1.0 wt % or more. Accordingly, the application to high-temperature water (primary cooling water) in the nuclear power field is unsuitable from the viewpoint of activation and the applicable scope is limited.
The ferrous-group shape memory alloys disclosed in the second prior art contain a higher Cr content with the purpose of improving corrosion resistance and the addition of Ti, Zr, V and Nb, and also high Mn content with the purpose of raising the shape-memorizing properties. This second prior art has the following problems. That is, firstly, though the Cr content is set at 10-17 wt %, the Cr content in the working example is less than 16 wt %. Accordingly, improvement in the intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high-temperature water expected as an effect of the addition of Cr are inadequate.
Because the Mn content is set at 6.0 wt % or more, the stress corrosion cracking resistance in high-temperature water is deteriorated by an increase of non-metal inclusions and the intergranular corrosion resistance in nitric acid is also deteriorated. Moreover, because the Co content is set at 2.0 wt % or more, the alloy is unsuitable from the viewpoint of activation for application to high-temperature water (primary cooling water) in the nuclear power field and its applicable scope is limited.
The alloys disclosed in the third prior art show excellent properties of stress corrosion cracking resistance. However, they can be roughly divided into alloys with an Si content of 2.9 wt % or less and alloys with an Si content of 3.8 wt % or more. Regarding the former, the intergranular corrosion resistance and stress corrosion cracking resistance in the said environment are inadequate because the Si content is 2.9 wt % or less. Regarding the latter, the shape memorizing property is inadequate because the ratio of the total content of austenite-forming elements to the total content of ferrite-forming elements is not appropriate.
For these reasons, the development of ferrous-group shape memory alloys with excellent shape-memorizing properties, intergranular corrosion resistance and stress corrosion cracking resistance that permit their application to nitric acid for nuclear fuel reprocessing plants and high-temperature water (primary cooling water) for light-water reactors is strongly desired. However, such ferrous-group shape memory alloys have not yet been achieved.